Correct Answer - Option 2 : By increasing the rotor resistance

**Concept:**

The torque equation of a three-phase induction motor is given by,

\(T = \frac{{180}}{{2\pi {N_s}}}\left( {\frac{{sE_2^2{R_2}}}{{\left( {R_2^2 + {s^2}X_2^2} \right)}}} \right)\)

Where Ns is the synchronous speed

E2 = rotor emf

R2 = rotor resistance

X2 = rotor reactance

s is the slip

By the above expression, we can say that the torque of an induction motor depends on rotor resistance, supply voltage, and slip.

From the design perspective, **the torque can be increased by increasing the rotor resistance.**

**Note:** While designing, if we keep the more rotor resistance, the torque will increase. But, we can't add extra resistance in the rotor circuit in the case of a squirrel cage motor.

The method of adding extra rotor resistance is applicable to W.R.I.M. due to wound rotor is short-circuited with slip rings, where external resistance is added through the brush whereas, in squirrel cage induction motor, the rotor is made of non-insulated copper conductors in rod or bar formats & short-circuited with end rings.